Membrane elements have long been regarded as efficient devices for separating components of fluid mixtures using, for example, ultrafiltration, microfiltration, nanofiltration and reverse osmosis processes. In a typical implementation, a pressurized fluid mixture is brought into contact with a membrane surface. Because of a difference in chemical potential and due to varying mass transport rates through the membrane, only some parts of a fluid mixture can pass through the membrane and a separation into constituent components is achieved.
In a typical spiral wound filtration cartridge, membrane sheets are wound around a centrally positioned porous tube. The fluid mixture, or feed, enters at one end of the cylindrical cartridge and travels through feed spacers positioned parallel to and between the membrane sheets. Separation occurs at the membrane-fluid interface; part of the fluid, called the permeate, passes through the membrane layer while the rest of the mixture remains on the opposite side of the membrane as more highly concentrated feed. The permeate stream travels in an inwardly spiraling radial direction until it passes through the walls of the central tube for recovery from one or both ends of the central tube. (see U.S. Pat. Nos. 4,235,723, 3,367,504, 3,504,796, 3,493,496, and 3,417,870.)
These spiral wound filtration cartridges are typically placed in a fiberglass or stainless steel pressure housing that provides mechanical strength to withstand the high feed pressures required for operation. It is undesirable for feed to flow uncontrolled through the annular space between the cartridge and the pressure housing because such uncontrolled bypass flow reduces the volume of feed fluid that is forced through the filtration element. This has two negative results: First, it reduces the volume of feed fluid that can be filtered on any one use of the filtration device. Second, the decreased feed flow reduces the hydrodynamic turbulence within the membrane filter and thus decreases the salt rejection efficiency. One method for preventing such bypass flow is to use brine seals to seal the outside of the cartridge to the inside of the pressure housing. However, an area of stagnant water can form in the annular space behind these brine seals and bacteria may grow in this stagnant water. Bacterial growth is unacceptable in sterile applications, such as applications involving food or medicine, in which these filters are sometimes used.
It has been proposed to provide a controlled bypass flow of feed within the annular space to prevent this bacterial growth. U.S. Pat. No. 4,301,013 discloses the use of a tight fitting open mesh within the annular space to control the bypass flow. It has also been proposed in U.S. Pat. No. 4,548,714 to wrap the cartridge with a leaf of feed spacer to provide for controlled fluid flow around the cartridge. U.S. Pat. No. 4,906,372 discloses a seamless porous rigid sleeve around the cartridge which separates the cartridge from the housing and provides a small, controlled bypass flow. All of these proposals require devices which must be manufactured within precise tolerances to provide a tight fit within the housing. Precise tolerances are difficult to produce with the materials and designs suggested in these patents. Extra-precision in manufacture is required to produce a user-friendly interchangeable cartridge. Further, none of the proposals provides mechanical support for the cartridge, which could be useful due to the high differential pressure which can develop within the pressure housing.
U.S. Pat. No. 5,128,037 describes a rigid shell surrounding the filtration cartridge that is sealed from the housing with a brine seal and which allows a small amount of impeded bypass flow through small holes or passageways. This design utilizes a brine seal and a relatively large annular space which are both potential stagnant areas that encourage bacterial growth.
Accordingly, it would be desirable to provide a filtration cartridge construction which allows a controlled feed bypass, close tolerances to the inner diameter of the pressure housing, mechanical rigidity to withstand the differential pressure forces, and a sanitary design that eliminates the brine seal and all stagnant areas.